The lettuce aphid (Nasonovia ribisnigri (Mosley)) is a major pest occurring in lettuce worldwide. The problem started to be severe for lettuce production in the 1970's in North Western Europe and spread rapidly all across Europe. Then, in the 1980's, the aphid was detected in Canada. Later on, the problem was reported in the USA (California and Arizona). More recently, the lettuce aphid was found in New Zealand and Australia.
Lettuce aphids can colonize lettuce plants at any plant stage and feed preferably from younger leaves. Large amount of aphids on the plant are able to reduce plant growth and deform the shape of the head so that the lettuce heads are then not marketable. The presence of high amounts of aphids in lettuce heads is a reason for retailers to refuse to buy lettuce from growers. At young plant stage, it is possible to control the lettuce aphid using insecticide. Several products were reported efficient to control aphid population. However, resistances to chemicals were reported in some aphid population. Moreover, at maturity, it is not possible to control aphids using insecticides as chemical products cannot enter into the lettuce head.
One of the most valuable strategies to control lettuce is genetic resistance. Extensive gene bank screening was performed and some Lactuca virosa accessions were found completely resistant to Nasonovia ribisnigri (Eenink and Dieleman, Euphytica 32(3), 691-695 (1982)). However, Lactuca virosa is in the third gene pool of the Lactuca germplasm according to the definition of Harlan. Therefore, these interspecific crosses are sterile, and the use of bridge species (as L. serriola) was necessary to transfer the resistance into L. sativa. Genetic analyses showed that the resistance to Nasonovia ribisnigri was controlled by a single dominant gene (Nr gene) in a L. sativa background.
However, breeders experienced that the release of varieties resistant to lettuce aphid was not straightforward. The Nr-resistance gene was found tightly linked to recessive genes conferring strong negative side-effects. Such plants showed a reduced growth, a pale green colour and a lack of fertility in seed set. Using large-sized progeny and molecular markers enabled lettuce breeders to find resistant recombinant plants without the negative side-effect phenotype (see EP-0 921 720). These resistant plants served as the source of the resistance gene that was not linked to the negative side-effect phenotype.
After this finding, the release of varieties resistant to Nasonovia became more and more important. The resistance became a major requirement for outdoor lettuce production for processing and also for fresh market.
In 2007, populations of lettuce aphids able to infect varieties resistant to Nasonovia ribisnigri were found in four distinct areas in Europe (France and Germany but also in Belgium and Austria). Four isolates (two from France and two from Germany) were analysed further by the Netherlands Inspection Service for Horticulture (also known as Naktuinbouw). They concluded the existence of a new Nasonovia ribisnigri biotype. This biotype is officially named Nr:1 and is able to overcome the Nr resistance gene. The Nr:0 biotype of the aphid can, however, still be efficiently controlled by the Nr gene.
The presence of Nr:1 Nasonovia ribisnigri organisms on lettuce has the same disadvantages as described above for Nr:0.
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